The thing you are missing, but since I am not there I cannot know the physical layout of the plumbing, is that the upstairs plumbing should go straight down the walls and so there is no logical reason why it would plug up. If this is true for your system, then the only reason for a stoppage could be a major problem with the installation. But it is possible, but not likely, that a wad of paper is caught at the point where the toilet drain turns down and that the tub and lavatory are also connected at that point. Good luck, you may need it.

: I know the clog is above the first floor - all the first floor drains that go into the same drainline are working fine. Water drains slowly from the tub - I can hear it dripping in the drain line coming down from the second floor. The toilet drains even slower.

: I plan on removing the toilet this evening and using a snake on the toilet drain (and I will go ahead and replace the wax seal while I'm at it, as well as the studs that hold the toilet down).